1. Field of the Invention
The present invention relates to an actuator, a protective cover for actuator, an actuator manufacturing method, and an optical deflector incorporating the actuator, and a two dimensional optical scanner and an image projector incorporating the optical deflector.
2. Description of the Prior Art
An actuator manufactured by micromachine technology (MEMS) has been improved in terms of power saving, downsizing, high-speed operation. A driver for the actuator made of a silicon wafer can be mass-produced at low price by semiconductor microfabrication technique.
FIG. 1 shows an example of such an actuator. In FIG. 1 an actuator 30 includes a rectangular frame element 31, a pair of beam elements (torsion bar) 32 as a rotary shaft, and a movable element 33. The pair of beam elements is attached to the frame element 31, and the movable element 33 is supported by the frame element 31 via the beam elements 32. A mirror face 33a, for example is formed on the movable element 33.
In the actuator 30 the movable element 33 is rotated by torsion of the beam elements as shown in FIGS. 2, 3. This actuator 30 can generate rotary oscillation at high speed with large amplitude by use of small energy from resonance.
Moreover, Japanese Patent Application Publication No. 2003-181800 discloses such an actuator 30 which includes meander beam elements 34 consisting of turnback portions 34a and straight portions 34b, and piezoelectric films formed on the meander beam elements as shown in FIGS. 4, 5, for example. This actuator with low spring stiffness can be largely inclined by alternatively bending neighboring straight portions 34b of the beam elements 32 in opposite directions.
That is, by providing the meander beam elements in the frame element 31, the actuator 30 can have a spring system equivalent to one made of a long straight torsion without an increase in the size of the frame element 31. Therefore, the spring stiffness of the beam elements can be lowered and the resonance frequency of the spring system can be lowered in low-speed operation. The actuator can be largely rotated by a small driving force even with no use of resonance.
Further, Japanese Patent Application Publication No. 2010-148265 discloses another actuator which includes a driver element on a meander oscillator to improve power supply efficiency to the driver element, for example.
Further, Japanese Patent Application Publication No. 2004-304856 discloses an actuator which includes a movable element, meander beam elements swingably supporting the movable element relative to the frame element, and a pectinate element fixed at a position to engage with the meander beam elements while the movable element is moved, for example.
This type of actuator, a MEMS device made of silicon as a brittle material, is likely to break down when receiving vertical force relative to the rotary axis.
Moreover, by use of this type of actuator in an optical scanner for two-dimensionally scanning an object with an optical beam to generate an image, lower stiffness of a spring system is preferable for the purpose of decreasing resonance frequency in sub scan direction and driving the actuator by small power even when no resonance occurs due to too high resonance frequency of a supporting spring system relative to a desired oscillation frequency. However, due to a low spring stiffness, the mechanical strength of the actuator is decreased so that it is more likely to be damaged or break down.
Moreover, the meander beam elements disclosed in the above documents achieve low spring stiffness in a limited space but have a problem in that applied with an impact, it is likely to move greatly compared with straight beam elements, therefore, more likely to break down.
FIG. 6 shows the normal rotation of the movable element 33 around the rotary axis O, for example. It is rotated as it is designed so that the moving area of the movable element 33 is limited and the actuator 30 is unlikely to break down.
Meanwhile, in FIGS. 7 to 8 the beam elements 32 (meander beam elements 34) are deformed and distorted, given unexpected force to the movable element 33 to move off of the rotary axis O. There is a possibility that the moving element 33 may hit a circuit element or the like disposed near the actuator 30 and the actuator 30 may be broken down.